[media] staging: media: davinci_vpfe: unlock on error in vpfe_reqbufs()

We should unlock before returning this error code in vpfe_reqbufs().

Fixes: 622897da ("[media] davinci: vpfe: add v4l2 video driver support")

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>